1. Field of the Invention
The present invention relates to a power-driven screw tightening apparatus having a torque control for detecting a screw tightening torque as a reactive force acting on an apparatus base to control the torque with which a screw is to be tightened.
2. Description of Prior Art
Quality control requires that automated screw tightening operation on various products such as industrial machines and electrical household appliances be effected by application of precisely controlled amount of torque dependent for, example, on the material into which screws are to be driven. To meet such precision requirement, there have been proposed and practiced many apparatus for controlling screw tightening torque.
One known torque control for screwdrivers powered by d.c. motors detects a motor armature current and de-energizes the motor for tightening screws with a desired torque when the motor armature current reaches a certain level, the motor armature current being proportional to the load imposed on the motor, or the tightening torque with which the screw is being fastened. When the head of the screw while being tightened engages the material into which the screw is driven, the motor armature is subjected to an abrupt speed reduction, but tends to rotate continuously due to inertia, thus keeping for a moment on producing a torque irrespective of the level of the armature current. At this time, the tightening torque is out of proportion with the armature current, and hence is likely to differ from a desired torque.
Japanese Laid-Open Patent Publication No. 54-36698 discloses a torque control device for power-driven screwdrivers which has a torque detector for detecting a screw tightening torque and a final decision unit for determining whether the torque falls within a predetermined range. With the disclosed arrangement, the tightening torque can be determined only after the screw has been fastened, and it is impossible to prevent screws in advance from being tightened with an excessive torque.